The brittleness and unreliability of ceramics in certain applications continue to present difficult and unsolved problems. The aerospace, automotive and aviation industries are but a few examples of industries that are searching for enabling technology to introduce new types of ceramics that are tough, flaw tolerant and exhibit graceful failure and creep resistance for both ambient and high temperature applications. Typical applications include components in turbine engines, cylinder sleeves for gasoline engines and structural components.
Recent trends in the ceramics research have been to reinforce brittle ceramic matrices with higher elastic modulus fibers, platelets, particulates, or whisker-shaped reinforcing elements embedded in the matrix. The reinforcing elements impart additional strength to the ceramic matrix. The additional strength is necessary to maintain the structural integrity of the ceramic matrices, particularly upon stress or shear-induced defects. These embedded reinforcing elements constitute large amounts of interfacial surface inside the ceramic composite. The deflection of a crack along such an interface causes separation of the interface due to the action of an impinging crack and is an important mechanism for enhancing the fracture toughness of the ceramic matrices. In a fiber-reinforced matrix, for example, the advancing crack can directly advance through the fiber potentially destroying the ceramic or it can debond along the interface and inhibit fiber failure. If debonding occurs, then the intact fibers will allow crack bridging and eventual fiber pull-out, thus giving rise to increased toughness of the composite.
It has been demonstrated that extensive fiber pullout can be induced by formation of a weak interphase layer between the fiber and the ceramic matrix. This has led to investigations of a variety of composites with coated fibers. This type of interfacial debonding mechanism of toughening has clearly been demonstrated in SiC materials reinforced with SiC fibers that were previously coated with a thin layer of compliant graphite (C) or boron nitride (BN). Toughness values of up to 30 MPa m1/2 have been reported for the graphite system when it operates under vacuum and ambient temperature. Alternating laminates of silicon carbide (SiC) and graphite have also functioned well in controlled oxygen deficient atmospheres. However, for high temperature applications (greater than 1000° C.), for extended use (greater than ten hours), or in air or oxidizing environments, both silicon carbide and graphite are chemically unstable and decompose to silica (SiO2) and gaseous species, e.g., carbon monoxide (CO). The resulting ceramic body is left porous, friable and weak.
Interphase layers are generally used to achieve the highest performance potential of ceramic matrix composites (CMC). Commercially available oxide CMCs (such as A/N720 from COI Ceramics, Inc.) do not rely on such interphases, but instead achieve crack deflection mechanisms using a porous, weak matrix. This weak matrix results in poor interlaminar properties, which tends to be a design-limiting feature. Higher matrix and interlaminar strengths may be achieved by increased densification, but at the sacrifice of crack deflection mechanisms and in-plane strain capability.
Interphase layers (such as monazite and porous zirconia) have been developed specifically for oxide matrix CMCs, but current application methods preclude their use in 3D reinforced fiber architectures.
Current methods for depositing coatings follow three paths. A first method involves the coating of fiber tows or fabrics. This method results in good quality, uniform coatings, but the coatings are not robust enough to withstand weaving or processing.
A second method involves the coating of fiber preforms. This method results in less uniform coatings; subsequent infiltration of matrix (especially within fiber bundles), and/or is inhibited by the presence of the coating.
A third method involves the introduction of an interphase material with the matrix. This method, also known as the Rockwell approach, mixes monazite and alumina, which compromises matrix strength since monazite is weak and prevents alumina particles from bonding.
Accordingly, what is needed is method for forming in-situ interphase layers in ceramic matrix composites that achieves uniform fiber coatings. Also what is needed is method for forming in-situ interphase layers in ceramic matrix composites that achieves high matrix strength in a three dimensional fiber preform.